As it is well known in the art of magnetic recording a desired high frequency audio bias recording or erase signal envelope level has to be switched on and off gradually, that is ramped-up or ramped-down, respectively, to eliminate an undesirable audible "pop" recorded on the magnetic medium. When utilized for example in video tape recorders, an optimum bias or erase level of an audio signal to be recorded has to be reached in a relatively short time for editing purposes, preferably within one video field or frame interval, that is within 30 milliseconds or less. Therefore, it is desirable to maintain a predetermined ramping period for respectively selected optimum signal levels within the operating range.
As an example, a prior art video tape recorder, type VPR-2, manufactured by Ampex Corporation, assignor of this patent application, utilizes for recording audio signals a ramped bias and erase signal generator circuit described in VPR-2 Video Production Recorder, Catalog No. 1809384-02, Page 10-7, issued in May 1980. Briefly, that prior art circuit (not shown) has an integrating operational amplifier for providing a desired ramped output voltage in reponse to an input voltage step. A voltage limiter connected to the integrator limits the output voltage therefrom to a value determined by a master bias level commonly applied as a D.C. reference voltage to all the recording channels. The limited ramped output signal from the integrator is then chopped by a solid state switch at a desired bias or erase signal frequency, and thereafter filtered to obtain a ramped high frequency sine wave signal. The thusly obtained signal is then attenuated by a potentiometer attenuator connected in the high frequency signal path in each respective channel to obtain respective desired optimum levels and thereafter the resulting attenuated signal is applied to a respective bias or erase current amplifier of that particular channel as it is well known in the art.
The above-described prior art circuit has the following disadvantages. Every time when the master bias level is changed the duration of the ramp also changes. Consequently during edits on tape when the "ramp-down" or "ramp-up" portion of a previously recorded information overlaps with a ramp portion of a newly recorded information going in the opposite direction, "holes" or "thumps" may be formed in the recorded signal envelope which in turn cause sudden disturbing changes in the edited audio signal volume. Another disadvantage is that the prior art circuit is not suitable for automatic control since the attenuator is located in the high frequency signal path, that is, at a remote location from the D.C. control circuit. The prior art circuit configuration cannot be readily adapted for use with printed wire boards. Moreover, as it is well known such potentiometers are exposed to signal level losses for example due to the temperature changes, or when utilized in multichannel recorders, to losses induced by interchannel crosstalk.